Experiments are performed on solutions of purified muscle proteins, especially myosin, its segments, and actin, with a view toward clarifying their interactions with each other as well as with ATP. Then this molecular information is transferred to experiments on fibers, the ultimate goal being to understand in physical terms the coupling of ATPase with the fiber structure so as to produce mechanical thrust. Because of the great sensitivity that it offers, and because of the high speed at which it is acquired our experiments are usually designed to generate fluorescence signals.